The invention relates to an apparatus for determining the injected fuel quantity in mixture compressing internal combustion engines by calculating the fuel required per stroke from the r.p.m. and the throttle valve position.
Mixture-compressing internal combustion engines must be supplied with the proper amount of fuel corresponding to the aspirated air quantity for each and every power stroke of the engine. The amount of fuel must be such that the combustion produces adequate power but operates without an excess of fuel which would result in an intolerably high degree of toxic components.
For these reasons, it is desired to supply a combustion fuel-air mixture which is either at the stoichiometric ratio, where the air number .lambda. = 1.0 or lies in a region in which there is an excess of air; one thus obtains a relatively lean mixture which is particularly suitable to reduce toxic exhaust gas components so as to permit compliance with constantly more rigorous requirements with respect to atmospheric purity. In order to correctly determine the quantity of fuel delivered to the engine per power stroke, i.e., to correctly adjust the duration of fuel injection when electromagnetically actuated injection valves are used, it is necessary to know exactly the air quantity which is being aspirated by the engine. This knowledge may be derived from measurement of the air flow rate in the induction tube of the engine, for example by means of a baffle plate which is displaced against a restoring force and serves to adjust appropriate metering means coupled thereto. Unfortunately, this a relatively expensive process which, furthermore, suffers from the inherent disadvantage that the increase of the engine torque is delayed with respect to the opening of the throttle valve due to the inertia of the air flow measuring member.
Instead of making an air flow measurement, it is also possible to fix the fuel injection duration on the basis of the engine r.p.m. and the induction tube pressure. By following the characteristic curve of an induction tube pressure sensor, the correct amount of fuel as a function of induction tube pressure for a particular r.p.m. may be determined.
However, induction tube pressure measurements are also quite complicated and, just as in the baffle plate measurement, additional sensors are required and the above-mentioned delay in the increase of engine torque is also present. A supplementary mechanism is also required to achieve a temporary enrichment during a change of the throttle valve position so as to obtain a good transition from one state to the next.
It is also known to derive the required fuel quantity from two relatively easily accessible engine data, namely the engine r.p.m. and the throttle valve angle .alpha.. When mechanical injection pumps are employed, the supplied fuel quantity is determined by a three-dimensional cam which adjusts for the proper fuel quantity as a function of a given r.p.m. and a given opening angle of the throttle valve.
It is also known to determine the fuel quantity to be injected electrically: by sensing the engine parameters r.p.m. and throttle valve angle, and this is a desirable method in principle because no additional sensors are required. The position of the throttle valve may be sensed relatively simply, for example with the aid of a suitable potentiometer and signals related to the engine r.p.m. may be derived from the ignition system or by simple sensing of an appropriate marker applied to the crankshaft with the aid of preferably an inductive transducer.
Unfortunately, the proper amount of fuel to be delivered to the internal combustion engine per power stroke is a relatively complicated function of the r.p.m. and the throttle valve position. Thus, each different type of engine has a particular and usually known set of characteristic curves which is exemplified by the diagram of FIG. 2 which will be discussed in more detail below. Due to the relatively complicated, nonlinear dependence, it has been considered to be impossible to generate the function corresponding to the proper fuel quantity to be injected by any reasonable means. In the function t.sub.i = f (.alpha., n), t.sub.i is the time during which the fuel is injected to a cylinder per power stroke and is therefore proportional to the fuel quantity Q. Since the above-mentioned function f is difficult to follow in a direct manner, a known circuit uses a low-pass filter in a pulse shaping circuit to transform it into a somewhat simpler function which is easier to follow, and this simpler function is subsequently multiplied by another r.p.m. dependent function. This known method also entails a substantial expense.